Composition and method for milling stainless steel and nickel base alloys



. machine. milling, grinding, etc. are accomplished very slowly with States Unite This invention relates to the shaping of metal parts and more particularly to a bath solution and process for shaping metal parts by controlled chemical dissolution.

In recent years it has increasingly become more apparent that conventional methods of machining parts, particularly aircraft parts, have become inadequate for obtaining present day needs. Various methods of fabricating aircraft parts have been employed in the past to obtain high strength, exceedingly lightweight articles. However, the conventional methods still do not readily permit the manufacture of parts having the most desirable strength to-weight ratios. Moreover, many of the metals currently being employed because of high strength and corrosion resistance at elevated temperatures are exceptionally hard and, therefore, difficult to conventionally Conventional machining operations, such as these relatively hard materials and, accordingly, machining such materials to the degree required to obtain optimum strength-toweight ratios is expensive and tedious.

In the manufacture of hollow turbine buckets for a jet engine, for example, corrosion-resistant, extremely hard alloys of nickel are frequently used. These turbine buckets need only have exceedingly thin walls for strength purposes and such thin walls are especially desired for efficient cooling of the blades. Moreover, the outer surface of the turbine bucket should be as smooth as possible to reduce friction with gaseous streams passing over the bucket. The fabrication of such a turbine bucket by conventional techniques is exceedingly difficult, time consuming and costly.

Parts made from metals, such as stainless steel and nickel base alloys, frequently are formed by casting, molding or the like wherein an undesirable projection frequently results at the parting line of the forming members. Similarly, during conventional machining operations a burr is likely to result on the periphery of the machined surface. These undesirable projections on the surface of the part must subsequently be removed by finishing operations. However, such finish operations are additionally time consuming and costly.

It is a primary object of this invention to provide a bath solution and process for treating a metal surface in such a manner as to dissolve the metal of the surface at a rapid rate while concurrently maintaining or improving thelsmoothness of the surface.

It is universally recognized that the surface finish of a part which is subjected to physical and thermal stresses should be exceptionally smooth. It is expected that the extremely smooth surface finishes are not only desirable When the part is to be used in a gas stream but, in most instances, necessary to obtain optimum resistance to fatigue and corrosion. Our invention provides a means whereby the surface of a part made of a metal, such as stainless steel and nickel base alloys, is subjected to a controlled etching which can be used to improve the smoothness or surface finish of the part to remove undesirable surface projections and to produce exceedingly thin structures.

Accordingly, it is a further object of our invention to provide a means whereby such materials can be produced ice economically and rapidly under commercial production conditions to produce optimum strength-to-weight ratios heretofore frequently unobtainable and to concurrently obtain surface finishes which were only laboriously obtained by conventional finishing techniques.

We have now found that a nickel base alloy can be satisfactorily machined by chemical dissolution in an aqueous bath containing ferric chloride, antimony trichloride, hydrochloric acid and nitric acid. By surface finish, as used herein, we refer to the degree of smoothness or roughness of a given surface as distinguished from the degree of reflectivity of a polished surface.

By the term controlled etching, as used herein, we refer to the etching of a metal surface in such a manner as to uniformly dissolve the metal without selectively etching grain boundaries or grain of the metal, thus at least maintaining the original surface condition. Such controlled etching will concurrently attack projections or peaks on the surface to a greater degree than recesses or valleys, thereby concurrently smoothing out or leveling the surface.

For the purposes of our invention by the term stainless steel we intend to encompass various ferrous base alloys containing approximately 10% or more chromium, such as alloys analogous to the SAE 300 and SAE 400 series stainless steels. References made herein to nickel base alloys are intended to include those metals containing more than 50%, by weight, nickel.

The following Table I contains examples of suitable high temperature, creep-resistant nickel base alloys which may be satisfactorily chemically machined in accordance with the present invention, the compositions being listed in percent, by weight:

Table I Example 1 Example 2 Exan1- Exam- Example 3 ple 4 ple 5 Carbon 1 0.15 0. 35-0. 45 1 0. 07 0.15 1 0. O8- Manganese -3 1 1 1 l 0.3-1 Chromium 19 alt 10 M Jlybdenu 10 Tungsten". Iron Vauadiurm Titanium Aluminum 7 Silicon 1 1 1 1 0.75 0. 65 0.5 Sulfur 1 0.03 1 0.01 Copp r 1 0.2 Nickel 1 Max. 2 Balance.

Although satisfactory results can be obtained with our solution when used to chemically machine a wide variety of both stainless steel and nickel base alloys, especially satisfactory results are obtainable when the solution is used to chemically machine a nickel base alloy, such as is disclosed in United States Patent No. 2,688,536, Webbere et al. Alloys of this character are extremely resistant to controlled etching but with our invention an extremely rapid but uniform metal removal is accomplished. This alloy comprises approximately 0.06% to 0.25% carbon, 13% to 17% chromium, 4% to 6% molybdenum, 8% to 12% iron, 1.5% to 3% titanium, 1% to 4% aluminum, 0.01% to 0.5% boron and the balance substantially all nickel. For some applications the aluminum content may be increased to approximately 6%, the molybdenum to about 7.0% and the iron content may be as low as 0.1% or as high as 35%. The alloy usually should not contain more than20% iron, however, Normally, manganese and silicon not in excess of 1% each are also included in the alloy.

As previously indicated, the bath of our invention encompasses an aqueous solution containing ferric chloride, hydrochloric acid, nitric acid and antimony trichloride. The ferric chloride is more conveniently obtained dissolved in an aqueous solution which has a density of approximately 42 Baum. In general, it is preferred to employ about 100 milliliters of a 42 Baum ferric chloride solution for every 640 milliliters of bath solution which is formed. However, satisfactory results may be obtained if the bath solution contains the equivalent of from about 13% to 18%, by volume, of the 42 Baum ferric chloride solution.

The amount of hydrochloric acid which is employed can only be varied to a rather limited extent when machining an alloy such as the nickel base alloy described in the aforementioned United States Patent No. 2,688,536, Webbere et a1. We prefer to employ about 300 milliliters of concentrated hydrochloric acid (specific gravity 1.19) for every 640 milliliters of bath solution which is formed. Generally, however, satisfactory bath solutions are obtained when using amounts of hydrochloric acid as low as about 42% or as high as 52% concentrated hydrochloric acid, by volume.

The presence of the nitric acid in our bath solution is essential to the reaction of the bath solution with especially non-reactive metals, such as the alloy described in the above-mentioned United States Patent No. 2,688,536, Webbere et al. Although only comparatively small amounts of nitric acid are required, these amounts are exceedingly important as the rate of reaction of the bath solution with the alloy being treated is a direct function of the amount of nitric acid present. For a nickel base alloy, such as the last-mentioned, 15 milliliters of concentrated nitric acid can be used in each 640 milliliters of bath solution. The optimum amount of nitric acid used not only depends upon the composition of the alloy but the relative amount of the alloy which is to be dis solved. Thus, for machining a small part in a large bath solution a lesser proportionate amount of nitric acid is required in the solution than when machining a comparatively larger part in a much smaller bath solution. At least about 0.8% of concentrated nitric acid is usually required to promote a satisfactory rate of reaction while, in certain instances, as high as 4%, by volume, can be used.

The amount of antimony trichloride which is required in the solution to maintain or improve the surface finish of the part being treated depends upon the specific composition of the alloy. Some nickel base alloys containing comparatively large amounts of boron, for example, do not dissolve uniformly without the presence of antimony trichloride. Although even small amounts of antimony trichloride assist in producing a more uniform dissolution, excessive amounts of antimony trichloride are not detrimental to the surface finish of the part. The minimum amount of antimony trichloride which is needed will depend upon the alloy being treated and the degree of smoothness desired Accordingly, we may prefer to employ at least small but effective amounts of antimony trichloride to as high as 25 grams antimony trichloride or more for every 640 milliliters of bath solution formed.

Of course the amount of water used in making the bath will depend upon the desired proportions of the other ingredients used in forming the bath. The abovementioned ferric chloride, hydrochloric acid and antimony trichloride are mixed together wtih a suitable amount of water without any of the nitric acid in the mixture. Excessive amounts of nitric acid may cause a deleteriously vigorous attack on the metal. This attack can be so violent as to virtually erupt the solution as a geyser whereupon the bath solution may be lost. Accordingly, we prefer to make up the solution without the nitric acid, immerse the part in the solution and subsequently add the nitric acid in small increments during the period in which the part is immersed in the machining solution. As the nitric acid is the regulator of the speed of the process, one can control the rate of chemical reaction by regulating the amount of nitric acid being added. The total amount of nitric acid which is generally added should be at least about 5 milliliters and in many instances as high as 25 milliliters for every 640 milliliters of bath solution, primarily depending upon the size of the part being machined.

When using a bath of our composition to form metal parts by chemical dissolution techniques, the part preferably is initially cast, forged or premachined to a preform configuration of desired measurements prior to the electrical dissolution treatment. It is generally desirable to form the part slightly oversize when no masking is employed so that during the dissolution thereof the part is reduced to finish dimensions. It is preferred, however, to form the part only as close to final desired specifications as is practical and economical by conventional techniques, keeping in mind that chemical dissolution tends to promote dimensional loss at peaks or projections. The optimum configuration of the preformed part is dependent upon the general structure of the part involved, the practicality of using extended amounts of conventional machining techniques prior to chemical machining, etc.

It is generally preferred to clean the metal surface prior to subjecting it to chemical machining treatments. Satisfactory results are obtainable when the part is cleaned by degreasing it in a trichloroethylene vapor at a temperature of approximately 180 F. in the normal and accepted manner. In some instances one of the many commercially available di-phase cleaners, which is a stable emulsion of an organic cleaner and an alkali cleaner, might be used. After degreasing, the part is dried and then, suitably supported, immersed in the machining bath solution.

The part is preferably positioned in the machining solution in such a manner as to avoid non-uniform dissolution of the surfaces. Gas which may be generated during the dissolution of the metal part can accumulate in recesses of horizontal areas of the part so as to interfere with uniform chemical dissolution of the entire surface. When machining an article having a planar configuration, such as a panel, it is desirable to support the panel in the machining solution in a vertical attitude.

On the other hand, articles of a more complicated configuration containing complex contours and recesses may not be suitably maintained in any position which will entirely inhibit collection of the generated gases and formation of gas pockets. For these and other types of articles it may be desirable to chemically machine the parts in a plurality of steps in which portions of the part are masked from the solution. For example, the top of such an article can be chemically machined while its lower surface is masked with a suitable stop-off material. When sufficient metal removal of the upper surface is obtained, the part is removed from the solution, rinsed and the stop-off removed to expose the protected surface. The machined surface is then masked and the part reimmersed for completion of the chemical machining of the part. Of course, on reimrnersion the part is inverted as compared to the previous machining operation so that the masked surface is on the bottom of the part.

The temperature at which the bath is operated is not especially critical and, in some instances, temperatures as low as room temperature can be used. Relatively high bath temperatures are preferred for commercial production use in order to reduce the resistance of the solution and pass more current through the solution per unit voltage. Accordingly, we have found that bath temperatures above F. are most desirable with the upper temperature limit, of course, being somewhat below the boiling point of the solution to avoid excessive vaporization.

The duration of the machining operation is primarily dependent upon the desired depth to which one desires to etch. In determining the preferred duration of etching one must give consideration, of course, to the fact that specifically exposed areas on the part, such as projections or peaks, tend to dissolve at a greater rate than recessed areas, contributing to dimensional loss. Dissolution durations limited by such dimensional loss can be ex tended by initially forming the part to compensate for faster metal removal on projections. Analogously, dimensional loss can be inhibited by using maskants and the like, as are Well known in the etching art.

Although this invention has been described in connection with certain specific examples thereof, no limitation is intended thereby except as defined in the appended claims.

We claim:

1. An aqueous bath for the chemical controlled etching of metals, said bath consisting essentially of about 14% to 18% of a 42 Baum aqueous ferric chloride solution, about 42% to 52% concentrated hydrochloric acid, about 0.5% to 4% concentrated nitric acid, all proportions by volume, and antimony trichloride to at least substantially maintain surface smoothness.

2. An aqueous bath for the chemical controlled etching of a metal from the group consisting of stainless steels and nickel base alloys, said bath consisting essentially of about 14% to 18% of a 42 Baum aqueous ferric chlo ride solution, about 42% to 52% concentrated hydrochloric acid, about 0.5% to 4% concentrated nitric acid, all proportions by volume, and from small but effective amounts to saturation of antimony trichloride.

3. The method of chemically machining a part of a metal selected from the group consisting of stainless steel and nickel base alloy, said method comprising applying to said part an aqueous bath solution containing the equivalent of about 14% to 18% of a 42 Baum aqueous ferric chloride solution, about 42% to 52% concentrated hydrochloric acid, all proportions by volume, and sufficient antimony trichloride to at least maintain surface smoothness and adding nitric acid to said solution while said solution is in contact with said part to regulate the rate of reaction of said solution with said part.

4. The method of chemically machining a part of a metal from the group consisting of stainless steels and nickel base alloys, said method comprising applying to said part an aqueous bath solution consisting essentially of about 14% to 18% of a 42 Baum aqueous ferric chloride solution, about 42% to 52% concentrated hydrochloric acid, all proportions by volume, and suflicient antimony trichloride to at least substantially maintain surface smoothness and adding nitric acid to said solution while said solution is in contact with said part to regulate the rate of reaction between said part and said bath solution.

5. The method of chemically machining a nickel base alloy part, said method comprising applying to said part an aqueous bath solution consisting essentially of about 14% to 18% of a 42 Baum aqueous ferric chloride solution, about 42% to 52% concentrated hydrochloric acid, all proportions by volume, and suflicient antimony trichloride to at least substantially maintain surface smoothness and adding from small amounts up to about 4%, by volume, nitric acid to said solution while said solution is in contact with said part to regulate the rate of reaction of said bath solution with said part.

6. The method of chemically machining a part made of a metal consisting essentially, by Weight, of about 0.06% to 0.25% carbon, 13% to 17% chromium, 4% to 6% molybdenum, 1% to 6% aluminum, 1.5% to 3% titanium, iron not in excess of 20%, boron not in excess of 0.5% and the balance substantially all nickel, said method comprising applying to said part an aqeuous bath solution consisting essentially of 14% to 18% of a 42 Baum aqueous ferric chloride solution, about 42% to 52% concentrated hydrochloric acid, all proportions by volume, and suflicient antimony trichloride to at least substantially maintain surface smoothness, adding from small amounts up to about 4%, by volume, concentrated nitric acid to said solution While said solution is in contact with said part to regulate the rate of reaction between said part and said bath solution.

7. The method of chemically machining a part made of a metal from the group consisting of stainless steel and a nickel base alloy, said method comprising placing a surface of said part in contact with an aqueous bath solution containing the equivalent of about 14% to 18% of a 42 Baum aqueous ferric chloride solution, about 42% to 52% concentrated hydrochloric acid, at least about 0.8% nitric acid, all proportions by volume, and sufficient antimony trichloride to at least maintain surface smoothness.

References Cited in the file of this patent UNITED STATES PATENTS 2,172,421 Uhlig Sept. 12, 1939 2,266,430 Matthews et a1. Dec. 16, 1941 2,441,300 Bunte May 11, 1948 2,684,892 Saulnier July 27, 1954 2,890,944 Hays June 16, 1959 OTHER REFERENCES Moneypenny: Stainless Iron & Steel, page 512, 2nd ed. rev., cpw. 1931, publ. Chapman & Hall, London. (Copy in Div. 3.)

Metals Handbook, 1948, ed., page 395, Table II, Etch Bath 8 (A) and (B). (Copy in Div. 3.) 

3. THE METHOD OF CHEMICALLY MACHINING A PART OF A METAL SELECTED FROM THE GROUP CONSISTING OF STAINLESS STEEL AND NICKEL BASE ALLOY, SAID METHOD COMPRISING APPLYING TO SAID PART AN AQUEOUS BATH SOLUTION CONTAINING THE EQUIVALENT OF ABOUT 14% TO 18% OF A 42* BAUME'' AQUEOUS FERRIC CHLORIDE SOLUTION, ABOUT 42% TO 52% CONCENTRATED HYDROCHLORIC ACID, ALL PROPORTIONS BY VOLUME, AND SUFFICIENT ANTIMONY TRICHLORIDE TO AT LEAST MAINTAIN SURFACE SMOOTHNESS AND ADDING NITRIC ACID TO SAID SOLUTION WHILE SAID SOLUTION IS IN CONTACT WITH SAID PART TO REGULATE THE RATE OF REACTION OF SAID SOLUTION WITH SAID PART. 